This invention relates generally to the casting of metal sheets and is particularly directed to the casting of metal sheets between counter rotating rollers.
Steel making occupies a central economic role and represents a significant fraction of the energy consumption of many industrialized nations. The bulk of steel making operations involves the production of steel plate and sheet. Present steel mill practice typically produces thin steel sheets by pouring liquid steel into a mold, whereupon the liquid steel solidifies upon contact with the cold mold surface. The solidified steel leaves the mold either as an ingot or as a continuous slab after it is cooled typically by water circulating within the mold wall during a solidification process. In either case, the solid steel is relatively thick, e.g., 6 inches or greater, and must be subsequently processed to reduce the thickness to the desired value and to improve metallurgical properties. The mold-formed steel is usually characterized by a surface roughened by defects, such as cold folds, liquation, hot tears and the like which result primarily from contact between the mold and the solidifying metallic shell. In addition, the steel ingot or slab thus cast also frequently exhibits considerable alloy segregation in its surface zone due to the initial cooling of the metal surface from the direct application of a coolant. Subsequent fabrication steps, such as rolling, extruding, forging and the like, usually require the scalping of the ingot or slab prior to working to remove both the surface defects as well as the alloy deficient zone adjacent to its surface. These additional steps, of course, increase the complexity and expense of steel production.
Steel slab thickness reduction is accomplished by a rolling mill which is very capital intensive and consumes large amounts of energy. The rolling process therefore contributes substantially to the cost of the steel sheet. In a typical installation, a 10 inch thick steel slab must be manipulated by at least ten rolling machines to reduce its thickness. The rolling mill may extend as much as one-half mile and cost as much as $500 million.
Another approach to forming thin metal sheets involves casting into approximately the final desired shape. Compared to current practice, a large reduction in steel sheet total cost and in the energy required for its production could be achieved if the sheets could be cast in near net shape, i.e. in shape and size closely approximating the final desired product. This would reduce the rolling mill operation and would result in a large savings in energy. There are several technologies currently under development which attempt to achieve these advantages by forming the steel sheets in the casting process. While some of the approaches under investigation use electromagnetic energy, all of these approaches use a solid mold on one or both sides of the sheet. One disadvantage of a solid mold is that contact between the molten metal and the solid mold wall often produce an undesirable surface finish which requires subsequent processing to correct as pointed out above.
Previous inventions have employed electromagnetic fields as a substitute for the solid molds. For example, the use of electromagnetic levitation techniques has been employed for some time in the aluminum industry. The practice there is to use electromagnetic fields to contain the top inch or so of a large, thick ingot. The molten aluminum is cooled and solidified before it touches any mechanical support. Examples of this approach can be found in U.S. Pat. Nos. 3,467,166 to Getselev, 4,161,206 to Yarwood et al, and 4,375,234 to Pryor. Also, U.S. Pat. No. 4,678,024 to Hull, et al, was directed toward use of alternating electromagnetic fields to levitate an entire sheet of molten metal for horizontal casting.
One of the difficulties associated with the use of electromagnetic fields as a substitute for solid wall molds is shaping the electromagnetic field into a uniform plane. Another difficulty with the use of electromagnetic fields for casting molds is that the electromagnetic field required to contain an entire sheet of molten metal is so strong that it causes large eddy currents in the molten metal which heat the metal thereby inhibiting solidification.
Another approach under consideration by the steel industry to reduce processing involves roller casting of sheets of steel. This method was originally invented by H. Bessemer over 100 years ago as described in British patent nos. 11,317 (1847) and 49,053 (1857) and a paper to the Iron and Steel Institutes, U.K. (October 1891). This method produces steel sheets by pouring molten steel between counter rotating twin-rollers. The rollers are separated by a gap. The twin-rollers contain the molten metal between the rollers. Mechanical seals are necessary to contain the molten metal on the ends of the rollers. The rollers can be made from a metal with high thermal conductivity, such as copper or copper alloys, and water cooled in order to solidify the skin of the molten metal before it leaves the gap between the rollers. The metal leaves the rollers in the form of a strip or sheet. This sheet can be further cooled by water or air supplied by jets or other suitable means. This method has the drawback that the mechanical seals used to contain the molten metal at the roller edges are in physical contact with both the rotating rollers and molten metal and therefore subject to wear, leaking, clogging, freezing and large thermal gradients. Furthermore, contact between the mechanical seals and the solidifying metal can cause irregularities along the edges of sheets cast in this manner thereby offsetting the advantages of the roller method.
The present invention overcomes the problems of roller casting and electromagnetic casting with a novel design which features the advantages of both roller casting and electromagnetic casting while overcoming the obstacles associated with both those methods.
Accordingly, it is an object of the present invention to provide an improved method and arrangement for casting thin metal sheets.
It is another object of the present invention to produce thin metal sheets which require little or no subsequent rolling after the sheet is cast.
Yet another object of the present invention is to reduce the cost and complexity of casting thin metal sheets.
A still further object of the present invention is to produce thin metal sheets using less energy.
Still another object of the present invention is to produce a metal product having good metallurgical properties and surface characteristics as it leaves the caster.
Another object of this invention is to provide for continuous roller casting of metal sheets.
It is still another object of this invention to provide containment of a pool of molten metal between twin-roller casters without sidewalls that make physical contact with the rollers.
A further objective of this invention is to prevent a pool of molten metal from flowing out the ends of counter rotating rollers by means of a shaped vertical alternating magnetic field.
A further objective of this invention is to provide an electromagnetic stopper or seal that is capable of preventing or regulating the flow of a molten metal in a horizontal direction.
Another object of the present invention is to cast molten metal in such a manner that the surface skin solidifies with little frictional contact with a mold.
An additional object of the present invention is to electromagnetically cast metal sheet with a minimum of electromagnetic heating of the molten and solid metal.
Another object of the present invention is to provide a system and method which is particularly adapted for the continuous casting of thin sheets of steel.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.